1. Technical Field
The embodiments herein generally relate to circuit design, and, more particularly, to voltage to current converters.
2. Description of the Related Art
Currently known circuit topologies for voltage to current converters suffer from poor linearity performance. Such linearity performance is because devices within circuits that interface with input serve two roles at the same time; sense the input signal and carry the output signal current. Since most silicon process technologies offer devices with a highly non-linear input-output relationship such as CMOS or BIPOLAR, using devices for both sensing input voltage signal and current conversion results in poor linearity of such devices. For example, a CMOS device's output current versus its input gate to source voltage follows a square law relationship and a BIPOLAR device follows an exponential relationship that is highly non-linear in nature.
In high dynamic range continuous time Sigma Delta Analog-to-Digital Converters (ΣΔADCs) the obtainable signal resolution when it is quantization limited is proportional to the clock frequency that the loop filter can handle. Voltage to current converter stages are inherently faster than operational amplifiers (OPAMP) so there is a great interest to using voltage to current converters and capacitors to build the loop filter's integrators instead of resistors, capacitors, and OPAMPs. However, all current voltage to current converter topologies suffer from the fact that they have poor linearity performance compared to OPAMPs, even when signal swing is moderate (for example, 200 mV peak to peak differential). Poor linearity poses a bottleneck for performance improvement as improvements to the signal resolution are sought, which is usually defined as ENOB (effective number of bits) or SINAD (Signal to Noise and Distortion). Improvements are made to the signal resolution by using voltage to current converters and capacitors to build integrators. When voltage to current converters and capacitors are used to increase the system clock frequency for better quantization noise performance, the poor linearity caused by voltage to current converters will cause more distortion. The signal-to-noise ratio (SNR) and distortion will not improve due to the lower quantization noise because the distortion becomes the dominant factor.
Also, in many of today's radio frequency (RF)/wireless applications, filters that can operate at high frequencies are useful to filter out blockers and images, etc. However, if such filters have poor linearity, distortion introduced in such filters is likely to overwhelm the benefit of using them.